400 is higher than any challah recipe I have used, I start at 375 and then turn down to 350.Maybe your oven is of temp, too and maybe it is not a terrific recipe! Cover means loosely with foil usually.

I would recommend using a lower temperature, too. I start about 375 and keep an eye on it. If it starts getting too dark, I'd turn it down by 25 degrees. I'd test for doneness by internal temperature in that case over time.

The Challah recipe I use (and the one I teach) bakes in a 350 degree oven for 10 minutes, then rotate and bake another 15 minutes until the bread is nicely browned and the center temperature of the loaf registers 195 degrees. If you'd like to try the recipe, click here:http://www.flournwater.com/food_010.htm